In recent years, with the appearance of high-speed Internet and various multimedia services, a wavelength division multiplexing (WDM) optical communication system has been actively studied in order to provide a massive amount of information.
In the WDM optical communication system, a technology that integrates several optical waveguides corresponding to respective channels in parallel to be implemented at a low cost is required so as to process optical signals of the channels having different wavelengths at a receiver and a transmitter.
In order to reduce costs, it is important to integrate optical active devices such as an optical transmission device, a photodiode and an optical amplifier and optical waveguide devices such as an arrayed waveguide grating (AWG) and an array type variable optical attenuator (VOA). Such optical integration technologies are classified into a monolithic integration technology that implements and integrates an optical active device and an optical waveguide as an optical semiconductor formed of a single material and a planar lightwave circuit hybrid integration technology that integrates an optical active device on a different type of planar lightwave circuit (PLC) platform using flip-chip bonding.
The monolithic integration technology has many limitations in implementing a low-cost optical integrated device due to problems in optimization, reproducibility and yield of each optical device.
Meanwhile, the planar lightwave circuit hybrid integration technology can be implemented with high yield at a low cost since the optical active device and the optical waveguide device each of which is optimized are hybrid-integrated.
However, in the planar lightwave circuit hybrid integration technology, since the optical active device and the optical waveguide device are hybrid-integrated by a flip-chip bonding method, a coupling loss between the optical waveguide device and the optical active device occurs and particularly, there is a high probability of difference in optical loss among channels when an array of optical transmission devices and an array of optical waveguides are hybrid-integrated. Therefore, in order to uniformly maintain the intensity of output light of respective channels in a multi-channel device, a method of monitoring light intensity on an optical waveguide of a planar lightwave circuit is required instead of a method of monitoring light generated from an optical transmission device at a rear end of the optical transmission device.
In the case of a singular planar optical waveguide device such as an AWG and an array type variable optical attenuator, not a planar lightwave circuit device in which an optical active device is hybrid-integrated, intensity of optical signals transmitted to respective channels generally varies for each optical waveguide channel due to an optical coupling loss between channel waveguides and optical fibers, an amplification characteristic for each optical wavelength and a difference in optical transmission paths.
Therefore, in order to exactly transmit signals of multi-channels, a means of readjusting optical signals of channels having different intensity to have uniform intensity is required. In order to readjust the intensity of the optical signal for each of the channels, first, it is required to exactly measure the intensity of the optical signal of each of the optical waveguide.
FIG. 1 is a plan view and a cross-sectional view of an optical waveguide platform with hybrid-integrated optical transmission device and monitoring photodiode in the related art.
Describing a method of manufacturing an optical waveguide platform in the related art, as shown in FIG. 1B, a lower cladding layer 101 and a core layer 102 of an optical waveguide are deposited on a substrate 100. In this case, the substrate 100 may be a silicon substrate or a quartz substrate.
Next, a waveguide pattern is formed on the core layer 102 using photolithography and a dry etching method. An upper cladding layer 103 is deposited on the etched core layer 102 to form a PLC optical waveguide 20. In this case, the lower cladding layer 101, the core layer 102 and the upper cladding layer 103 of the PLC optical waveguide 20 may be formed of silica or a polymer.
In a PLC having the optical waveguide 20, a trench is formed using the photolithography and dry etching method to form a terrace 104 to which an optical transmission device 30 is to be flip-chip bonded, and a terrace 105 to which a monitoring photodiode 40 is flip-chip bonded. In this case, depths of the terraces 104 and 105 are determined so that the height of the terrace 104 is set for the optical transmission device 30 to be mounted and a core layer 111 of the optical transmission device to have the same height and the height of the terrace 105 is set for a monitoring photodiode 40 to be mounted and a core layer 121 to have the same height, and then the terraces 104 and 105 are etched at the determined depths.
As shown in FIG. 1B, an upper cladding layer 110 of the optical transmission device 30 and an upper cladding layer 120 of the monitoring photodiode 40, which are optimized, have different thicknesses and the core layer 111 of the optical transmission device and the core layer 121 of the monitoring photodiode, which are optimized, have different thicknesses.
Since the etching depth of the terrace 104 of the optical transmission device 30 and the etching depth of the terrace 105 of the monitoring photodiode 40 are different from each other as described above, a process of manufacturing an optical waveguide platform becomes complicated.
When the etching depth of the terrace 104 of the optical transmission device 30 and the etching depth of the terrace 105 of the monitoring photodiode 40 are not set differently, thicknesses of a metal line 130, a solder 131 and a flip-chip bonding pad 132 need to be differently set in accordance with the optical transmission device 30 and the monitoring photodiode 40.
The metal line 130 is formed of Cr/Ni/Au, NiCr/Ni/Au, Ti/Ni/Au, Ni/Au and Ti/Pt/Au, and the solder 131 is formed of metal or a metal compound having a low melting point, such as AuSn and In.
The flip-chip bonding pad 132 is formed of Cr/Ni/Au, NiCr/Ni/Au, Ti/Ni/Au, Ni/Au and Ti/Pt/Au.
Meanwhile, as shown in FIG. 1A, when the optical transmission device 30 is flip-chip bonded to the optical waveguide platform, and the monitoring photodiode 40 is flip-chip bonded to a rear end of the optical transmission device 30, an optical coupling loss between the optical transmission device 30 and the monitoring photodiode 40 occurs and thus increases as the distance between the two devices increases. Therefore, in order to minimize the optical coupling loss, the distance between the optical transmission device 30 and the monitoring photodiode 40 is typically set very densely to be 50 μm or less, which makes it difficult to dispose the metal line 50 on the PLC platform. Considering Joule's heat and impedance matching, the metal line 50 has a line width of at least 50 μm.
As shown in FIG. 1A, an array of the optical transmission devices 30 may be bonded to the PLC platform by flip-chip bonding once or a single optical transmission device 30 may be bonded to the PLC optical waveguide by flip-chip bonding several times. Both cases have a problem in that the coupling loss between the optical transmission device and the PLC optical waveguide may vary for each channel by misaligning due to horizontal and vertical directions or inclination during a flip-chip bonding process.